Prosthetic hip joints are routinely used to replace a patient's hip joint when that joint is damaged. Many tens of thousands of such hip replacement operations are performed each year around the world.
The prosthetic hip joint typically comprises an acetabular prosthetic socket or cup, which is placed in the hip and a femoral head, which fits into the cup.
It has long since been appreciated that the acetabular cup substituting for the acetabulum has to be inserted into the pelvis in the correct orientation. However, the extent to which the acetabular cup is often incorrectly oriented has only been established in recent years.
The techniques used by orthopaedic surgeons for orienting the prosthetic acetabular cup range from the most basic “eyeballing”, where the surgeon uses his experience and orients the cup as he thinks best, with no particular equipment to assist him in establishing that the cup is correctly oriented, to very complex navigation systems, with basic spirit levels and laser beams mounted on the introducer falling in between.
Examples of different navigation systems are described in U.S. Pat. Nos. 5,141,512, 6,711,431 and US 2009/0171370.
U.S. Pat. No. 5,141,512 describes a navigation system including a light source having an angle adjusting element, and three foot portions each fixed to a respective point of the pelvis. An angle adjusting element controls the direction of a light beam emitted from a light source, so that the direction corresponds to the insertion orientation of the prosthetic acetabular cup. This system does not take account of the any change in position of the patient during surgery.
U.S. Pat. No. 6,711,431 describes a system that defines a patient's pelvic plane with reference to at least three pelvic points, and traces a pelvic tracking marker, fixable to the pelvic bone, by using a location tracking device, which tracks the orientation of the pelvic plane in real time. The system can trace the patient's pelvic plane regardless of variation in the patient's pelvic plane position. However, this system is physically large and expensive.
US 2009/0171370 describes a navigation system for the fitting of a prosthetic acetabular cup which includes a mechanism capable of indicating the pelvic plane regardless of variation in the patient's pelvic position during surgery. The mechanism includes a y-shaped member mounting three probes, each of which rests on a different point on the pelvis.
Other simpler devices for assisting the surgeon in orienting the acetabular cup comprise instruments that are mounted on the introducer (the device that is used to force the acetabular cup into the pelvis).
For example, U.S. Pat. No. 6,743,235 describes an introducer that mounts a lever to which are attached a one dimensional spirit level and laser, the spirit level being fixed with respect to the lever and the laser being pivotably mounted on the lever. The spirit level is mounted on the lever such that when the introducer is positioned with its longitudinal axis aligned at 45 degrees to the horizontal, the spirit level is horizontal. The surgeon uses the laser to guide him in adjusting the extent of antiversion of the prosthetic acetabular cup by aligning the laser beam with respect to the acetabulum.
U.S. Pat. No. 6,214,014 describes a system which uses a goniometer to engage with bone portions of the pelvis measure a reference angle. The system allows the surgeon to adjust the goniometer to a desired offset. A laser beam aligned with the goniometer is then fired at a wall and a mark made on the wall. The surgeon then removes the goniometer. The system includes an introducer having a laser mounted thereon. When the laser beam is aligned with the mark on the wall the introducer is correctly oriented.
None of the known techniques of orienting the prosthetic acetabular cup is satisfactory. The more complex and hence more accurate devices are physically large, resulting in cramped conditions in smaller operating theatres, and are expensive.
The more basic instrument aids give the orthopaedic surgeon some guidance beyond simply lining up the prosthetic acetabular cup by eye. However, they do not provide for the cup to position with reference to the position of the hip joint in the standing position.
The hip is designed to function with the person in the standing position. However, hip replacement operations are performed with the patient lying down, usually on his side, with the hip to be replaced uppermost. The position occupied by the pelvis when the patient is lying down is different to the position occupied by the pelvis when the person is standing.
It would be desirable to provide an apparatus that would allow the prosthetic acetabular cup to be oriented as closely as possible to the desired orientation.
The prosthetic acetabular cup must be oriented in two planes. With the patient in the standing position it is desirable that the face of the cup should be aligned at 45 degrees to the vertical and that the face of the cup should lie at approximately 15 degrees of antiversion with respect to a vertical plane extending through the femoral balls.